CQ: do not force HEAD revision for chromium_trybot builders

commit-queue/projects.py

Index: commit-queue/projects.py
===================================================================
--- commit-queue/projects.py	(revision 247015)
+++ commit-queue/projects.py	(working copy)
@@ -459,11 +459,15 @@
 
     # Experimental recipe-based Chromium trybots. To avoid possible capacity
     # problems, only enable for a small percentage of try runs.
-    # Disable as emergency measure - crbug.com/334681
+    #
+    # Note how we pass revision=None . In presence of safesync_url
+    # this makes a difference: no revision uses safesync_url,
+    # while HEAD ignores it and always fetches latest revision.
     verifiers.append(
         trigger_experimental_try_job.TriggerExperimentalTryJobVerifier(
             context_obj,
             percentage=0.25,
+            revision=None,
             try_job_description={
                 'linux_chromium_rel': ['defaulttests'],
                 'mac_chromium_rel': ['defaulttests'],